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Calculating Speed (AQA GCSE Physics)
Revision Note
Calculating Speed
- For objects that are moving with a constant speed, use the equation below to calculate the speed:
- Where:
- v = speed in metres per second (m/s)
- s = distance travelled in metres (m)
- t = time taken in seconds (s)
Worked example
Planes fly at typical speeds of around 250 m/s. Calculate the distance travelled by a plane moving at this speed for 2 hours.
Step 1: List the known quantities
- Speed, v = 250 m/s
- Time, t = 2 hours
Step 2: Write the relevant equation
Step 3: Rearrange for distance travelled, s
s = v × t
Step 4: Convert any units
- The time given in the question is not in standard units
- Convert 2 hours into seconds:
2 hours = 2 × 60 ×60 = 7200 s
Step 5: Substitute in the values for speed and time
s = 250 × 7200 = 1 800 000 m
Calculating Average Speed
- In some cases, the speed of a moving object is not constant
- For example, the object might be moving faster or slower at certain moments in time (accelerating and decelerating)
- Because its speed is not constant, it is moving with non-uniform motion
- The equation for calculating the average speed for non-uniform motion is:
- This can be rearranged with the help of a formula triangle as shown:
Average speed, total distance, time formula triangle
Worked example
Florence Griffith Joyner set the women’s 100 m world record in 1988, with a time of 10.49 s. Calculate her average speed during the race.
Step 1: List the known quantities
- Distance, s = 100 m
- Time, t = 10.49 s
Step 2: Write the relevant equation
- Sprinters typically speed up out of the blocks up to some maximum speed
- Because Florence’s speed changes over the race, we can calculate her average speed using the equation:
average speed = total distance ÷ time taken
Step 3: Check any unit conversions
- Check that all quantities given in the question are in standard units
- In this example, they are all in standard units
Step 4: Substitute the values for total distance and time
Average speed = 100 ÷ 10.49 = 9.53288... = 9.53 m/s
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